A combination of muscle stiffness and spinal motion segment stiffness protect the lumbar spine against the possibility of buckling instabilities. This is an application to support continued research (of R01 AR 44119) analyzing and quantifying the degree of muscle stiffness required for stability under a variety of loading conditions and spinal postures. Previous analyses have predicted that antagonistic muscle activation would increase stability, and the existence of such antagonistic activity has been confirmed in experimental studies documenting muscle activation electromyographically. The overall hypothesis is that impaired muscular control might predispose the lumbar spine to injurious buckling episodes. The proposed continuation studies will focus on four main areas of work: (1) human subject experiments to characterize muscle activation patterns under gradually increasing efforts and under perturbed loading conditions, and to measure driving point stiffness of the trunk, from which muscle stiffness will be deduced under differing activation conditions; (2) Experimental quantification of the stiffness of lumbar spinal motion segments (with and without intact posterior elements) under physiological conditions of axial compressive loading and surrounding fluid medium. (3 Simulations of these experiments will be performed using a new analytical model which calculates the muscle activation pattern required to optimize a variable combination of five cost function components, namely the global motion of the spine, intervertebral displacements, loading of intervertebral motion segments, the sum of cubed muscle stress and stability (via the magnitude of eigenvalues of the system stiffness). Some of the experimental data from human subject and motion segment studies will be used to assign values to parameters in the model, other data will be used to validate the model. (4) Longitudinal studies of muscle function in subjects with intermittent low back pain will indicate whether these individuals have muscle activation behaviors conducive to spinal instability. These studies will help to define the relative roles of muscle stiffness and spinal stiffness in stabilizing the spinal column. They will identify the conditions under which (1) reduced motion segment stiffness, (2) reduced muscular stiffness or (3) abnormal patterns of muscle recruitment might place the lumbar spine at risk for self-injurious buckling episodes under apparently benign external loading conditions. In addition, the specific roles of individual muscles will be better elucidated, which will be helpful in guiding therapy for people with low back dysfunction.